The speed of a light wave is a measurement of how far it travels in a certain time.
The speed of light is usually measured in metres per second (m/s).
Light travels through a vacuum at a bit less than 300,000 kilometres per second.
The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
Light travels through other media at lower speeds.
A vacuum is a region of space that contains no matter.
Matter is anything that has mass and occupies space by having volume.
When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
In other contexts, empty space is not considered to be a medium because it does not contain matter.
When light is described in terms of photons rather than waves the following points are important:
Photons are massless particles that travel at the speed of light.
Photons carry energy and momentum in quantized discrete units.
“Quantized discrete units” refers to the way energy and momentum are carried by photons.
In quantum mechanics, certain physical properties, such as energy and momentum, are quantized, meaning they can only take specific discrete values rather than a continuous range of values.
For photons, this means that their energy and momentum come in distinct, non-continuous packets or “units.”